Heat treatment of vinylidene chloride polymers



Patented Jan. 26, 1943 HEAT TREATMENT OF VINYLIDENE CHLORIDE POLYMERSJack L. Williams, Midland, Mich., assignor to The Dow Chemical Company,Midland, Mich, a corporation of Michigan No Drawing. ApplicationDecember 12, 1940,

Serial No. 369,869

8 Claims.

This invention relates to a. method whereby fabricated articles composedof polymeric vinylidene chloride may be treated to improve theirstrength properties.

The polymer of vinylidene chloride and many of its co-polymers,plasticized compositions thereof, and methods whereby the same may beproduced, are described in U. S. Patents Nos. 2,160,- 903-4 and2,160,931-948. Certain fabricated articles produced from vinylidenechloride polymers are claimed in the co-pending application of R. M.Wiley, Serial No. 211,055, filed May 31, 1938, now U. S. Patent No.2,233,442. The methods known heretofore whereby vinylidene chloridearticles may be treated to improve the strength include those describedby R. S. McClurg and D. L. Gibb in U. S. Patent No. 2,176,091 and by R.M. Wiley in U. S. Patents Nos. 2,183,602 and 2,205,449.

The present invention relates in particular to a method whereby thinsheets or films or filamentous articles such as threads, fibers,cordage, and the like, which have been previously stretched andoriented, suitably as described by R. M. Wiley in Patent No. 2,183,602,and in application Serial No. 211,055, may be treated further to improvethe tensile strength thereof. The vinylidene chloride compositions hereconcerned are those which contain normally crystalline vinylidenechloride polymers! This expression finds defimtion in the aforesaidapplication of R. M. Wiley and will be employed herein to designate thepolymer of vinylidene chloride and any of its copolymers which onexamination by X-ray diffraction methods are found to exhibitcrystalline characteristics. In the following description the termnormally crystalline vinylidene chloride polymers will be understood torefer not only to the polymers themselves but also to plasticcompositions containing the same, e. g., plasticized polymers, or thosewhich have been modified by the addition thereto of such other eflectmaterials as dyes, pigments, or agents to stabilize the polymers againstthe decompositional efiects of light or heat.

While it is a common property of normally crystalline vinylidenechloride polymers that they are capable of forming films or filaments ofgreater strength than those producible from other polymeric masses whichdo not exhibit crystallinity on X-ray examination, it is still an aim ofthose working with vinylidene chloride articles further to improve theirtensile strength. It is this aim which constitutes the object of thepres-- ent invention.

It has now been found that the foregoing object may be attained and thatthe tensile strength of previously stretched and oriented films orfilaments produced from normally crystalline vinylidene chloridepolymers may be materially improved by a simple aging and heat treatmentprolonged for from 6 hours to about 20 days, in a specific range ofmoderate temperatures between about C. and about 75 C. Thesetemperatures are considerably below the softening point of the normallycrystalline vinylidene chloride polymers, which usually lie in the rangeform about 120 C. to 190 C.

In accordance with the present invention, a filament or like articlewhich has been formed suitably by extrusion and stretching to provideorientation of the crystals therein, is heated in any suitable medium,which may be a gas, such as air, or a liquid inert to the polymer, suchas glycerine, at a temperature between about 45 C. and about 75 C. for aperiod, varying inversely as the temperature, in the range from 6 hoursto 20 days. It has not been found necessary in any instance to employlonger periods of heat treatment than about 20 days and, in general,little, if any, advantage accrues to heating periods extendingbeyond 15days, even when operating at temperatures such as C. near the lower endof the herein-claimed range. When the heat treatment is eifected, forexample, at, C., which is near the upper end of the claimed tem-Derature range, it is preferred not to heat beyond 5 days, and littleadvantage has been found to accrue to a heat treatment at thattemperature prolonged beyond 2 days. Noticeably advantageous results areobtained through the use of the present heat treatment, even for periodsas short as six hours. Preferred conditions are 50 C. for 1 to 15 daysor 70 C. for 6 hours to 2 days.

The invention will be illustrated with reference to the following datawhich were obtained on a number of samples of extruded and stretchedfilaments of normally crystalline vinylidene chloride polymers, whichfilaments had been prepared as described in Patent No. 2,183,602. Someof the polymers employed were prepared by the copolymerization ofvinylidene chloride and vinyl chloride, while others were prepared fromvarious mixtures of vinylidene chloride, ethyl acrylate, and methylmethacrylate. In each case the amount of vinylidene chloride in theco-polymer was greater than per cent and in most instances thevinylidene chloride constituted from to 92 per cent of the co-polymer.Each of the -p ymers reported in the following table had a softeningpoint above 120 C. and a decomposition temperature above 190. C. in theplasticized condition in which they were employed, each sample havingbeen treated with from 2 to 10 per cent of a stabilizing plasticizersuch as di-(alpha phenyl-ethyl) ether, or mixtures thereof withphenoxy-propylene oxide. The extruded and stretched samples were cutinto 40 inch lengths and wound on sample cards which were placed in anoven having an accurately controlled temperature. Samples were removedfor visual observation and mechanical tests at frequent intervals andthe data as to tensile strength are reported in the following tables.

TABLE 1 Age treating at 50 C'.Tensile strength in thousands of poundsper square inch Time, days Sample so 31 45 44 43.5 41 l 35 a7 40 41 4242.5 40 42 as 40 40 40 40 42 4o 29 a 31 4o 35 so 23 30 2s 29 29 so 23 a26 21 27 24 25 TABLE 11 Age treating at 70 C.Tensile strength inthousands of pounds per square inch Time, days Sample No.

1.. as 31 31 2 37 39 39 3.. as ,40 43 4.. 29 as 29 5 2% 29 30 a 23 25 25Similar observations made on other of the crystalline vinylidenechloride polymers confirm the general applicability and advantage of thepresent method as means for improving the tensile strength of films,filaments, fibers, thread, or twisted or woven cordage prepared fromextruded and stretched samples of these polymers. Polymers other thanthose hereinbefore specifically designated, capable of forming films,filaments, and like articles which may be subjected to theherein-claimed treatment, include copolymers of vinylidene chloride withstyrene, vinyl acetate, chloroallyl chloroacetate, vinyl cyanide, andthe like, in proportions which exhibit crystallinity on X-rayexamination.

Heat treatments carried out at 90 C. on numerous crystalline vinylidenechloride polymers, indicated this to be too high a temperature for safepractical operation while those carried out in the range from 45 to 75C. uniformly showed tensile strength improvements of several per cent,comparable with those shown in the foregoing tables. Room temperatureaging fails to produce the advantageous results here obtained. Thecombined age-heat treatment of the present invention by its verysimplicity is of great advantage and importance in the preparation offabricated articles of the type described of improved tensile strength.

I claim:

1. The method which comprises age-heat treating a previously stretchedarticle of a normally crystalline vinylidene chloride polymer, at atemperature between about C. and about 75 C., and for a period betweenabout 6 hours and about 20 days, to increase the tensile strength of thearticle. 7

2. The method which comprises age-heat treating a previously stretchedand oriented filament of a normally crystalline vinylidene chloridepolymer at a temperature between about 45 C. and about 75 C., and for aperiod between about 6 hours and about 20 days, to increase the tensilestrength of the filament.

3. The method which comprises age-heat treating a previously stretchedfilm of a normally crystalline vinylidene chloride polymer at atemperature between about 45 C. and 75 C., and for a period betweenabout 6 hours and about 20 days, to increase the tensile strength of thefilm.

4. The method which comprises age-heat treating a previously stretchedarticle of a normally crystalline vinylidene chloride polymer at atemperature near C. for a period of from 1 to 15 days, to increase thetensile strength of the article.

5. The method which comprises age-heat treating a previously stretchedarticle of a normally crystalline vinylidene chloride polymer at a.temperature near C. for a period of from 6 hours to 2 days, to increasethe tensile strength of the article.

6. The method claimed in claim 1, wherein the polymeric article ispredominantly a normally crystalline vinylidene chloride-vinyl chlorideco-polymer.

7. The method claimed in claim 1, wherein the polymeric article ispredominantly a normally crystalline vinylidene chloride-ethylacrylate-methyl methacrylate co-polymer.

8. The method claimed in claim 1, wherein the polymeric article is madeof a plasticized normallycrystalline vinylidene chloride polymer.

JACK L. WILLIAMS.

